Patent GB-679167-A (GB)
Title: Communication systems between two stations linked by television
IPC: G01S13/86
Filed: 1947-11-17
Abstract: 679,167. Electric control systems. RADIOINDUSTRIE SOC. ANON. Nov. 17, 1947. [Jan. 16, 1947] No. 30460/47. Class 40 (i). [Also in Groups XL (b) and XL (c)] A television transmission system, particularly between an airborne transmitter and a ground or carrier-borne receiver comprises additional apparatus for the transmission of indications or intelligence between the transmitter and receiver by means of suitably modulated pulse signals primarily generated at the receiver location and transmitted to the transmitter location, where they additionally effect synchronization of the television transmitter. In an application of the invention to a pulsed radar system line and frame television synchronising pulse signals generated at and transmitted from a fixed ground located station are received at a mobile station (e.g. aircraft) where they are employed to synchronise the operation of a television camera the picture signals generated by which are mixed with the received synchronising signals and retransmitted to the ground station where the picture signals are reproduced on a cathode-ray tube. The retransmitted line synchronising signals are also employed, at the ground station, to determine the distance between the two stations, as in conventional radar practice, the received line synchronising pulses being applied to a second cathode-ray tube to produce a 'blip' in a trace the start of which occurs at the instant of transmission of a line synchronising pulse. When the distance of the mobile station from the fixed station is such that several line synchronizing pulses are transmitted before a retransmitted pulse is received, selective identification of pulses at a lower repetition rate may be effected by varying a characteristic (e.g. height, length or sign) of selected pulses occurring at a sub-multiple of the line-synch frequency, the initiation of the trace being produced by transmission of the selected pulses and the other pulses employed to establish a reference scale. The general arrangement of the fixed and mobile stations is shown in Figs. 12 and 13 respectively. In Fig. 12 line and frame synch-pulses from generator 1 which are amplitude modulated by signals from a source 31 (e.g. for tele-control of apparatus at the mobile station), and radiated from aerial 3 are received at aerial 41 (Fig. 13) of the mobile (e.g. aircraft) station and after the tele-control modulation has been abstracted and fed to utilization apparatus (e.g. servo mechanisms) in unit 45, are employed to control the operation of a television camera 5. The received line synchronizing, signals are also fed to a modulator 48 in which they are modulated by a reference low frequency signal 50 and by one or more low-frequency signals which are harmonics of the reference signal, the phase of each of the second signals relative to the first signal being controlled respectively by the indication of measuring apparatus, such as 54, and the order of the harmonic serving to identify the particular apparatus. The line synchronising signals thus modulated are then combined with the picture signals from camera 5 and retransmitted by transmitter 46, retransmission of the frame synchronizing signals being necessary only when the distance between the two stations is very great. The line synch. pulses (or selected pulses thereof) are also retransmitted, with the picture modulation, by means of a responder 22 and the signals received by aerial 24 after reflection from the ground are again retransmitted by the transmitter 46. At the ground station the lowfrequency modulation of the pulses giving instrument indication, is abstracted and employed to operate appropriate indicating devices such as 40 and the picture and synch, signals are applied to a picture reproducing tube 7, the line synch. signals being also fed to a range indicating tube 9 the horizontal trace of which is triggered by pulses from generator 1 (or by selected pulses). The range is indicated on the tube 9 by the first 'blip', i.e. due to the directly retransmitted pulses, and the altitude by the interval between this 'blip' and a second 'blip' due to the second retransmission of pulses after reflection from the ground. These latter pulses also form a vertical line 28 on the screen of tube 7 the distance of which from the left hand edge corresponds to the altitude. The synchronising pulses may be modulated with telephony signals in addition to the tele-control and instrument indication signals. The tube 9 may be operated as a P.P.I. system the display on the screen of which is projected onto a transparent map of the locality in the vicinity of the fixed station. In an alternative arrangement a manually, or partly automatically, controlled light beam is arranged to illuminate a spot on a map which corresponds to the position of the mobile station, Fig. 9 (not shown).